In the photographic industry the need to protect an imaging element from dirt and dust, scratches and abrasion, and deposition of stains has long been recognized. Significant progress has been made in the prevention of dirt and dust attraction through the use of antistatic layers in imaging elements. Improved protective overcoats have reduced the propensity for imaging elements to be scratched or abraded during manufacture and use. However, there is still a need to improve the stain resistance of imaging elements.
A wide variety of substances may adsorb onto or absorb into either the front or back surface of imaging elements and cause a permanent stain that degrades image quality. The deposition of these stain causing substances onto an imaging element may occur in many different ways. For example, dirt, fingerprints, and grease may be deposited onto the imaging element during handling. An imaging element may be stained when it comes in contact with a dirty surface or as a result of an accidental spill from, for example, a liquid drink such as coffee or soda. Other stains may be deposited onto a wet photographic element during film processing. For example, a tar-like material which is derived mostly from polymeric oxidized developer and which may be present at the surface of or on the walls of film processing solution tanks may be deposited during film processing. This tar may adhere to or diffuse into the surface layer of the imaging element and cause an extremely difficult to remove, brown-colored stain.
The treatment of articles such as textiles and food containers with soil and stain resistant compositions is well known. For example, U.S. Pat. Nos. 3,574,791 and 3,728,151 disclose block or graft copolymers which have two different segments, one of which is highly fluorinated and oleophobic and the other of which is hydrophilic. U.S. Pat. No. 4,579,924 describes fluorochemical copolymers useful as paper making additives which impart oil and water repellancy and food stain resistance to ovenable paperboard food containers. U.S. Pat. No. 5,350,795 describes aqueous and oil repellent compositions which cure at ambient temperature. The compositions comprise an aqueous solution or dispersion of a fluorochemical acrylate copolymer and a polyalkoxylated polyurethane having pendant perfluoroalkyl groups. U.S. Pat. No. Re. 34,348 discloses stain resistant compositions containing fluorinated polymers derived from acrylamide-functional monomers. Fluorocarbon containing coatings for hard tissue and surfaces of the oral environment that reduce staining and adhesion of bacteria and proteinaceous substances are described in U.S. Pat. Nos. 5,662,887 and 5,607,663.
For an imaging element the requirements for a stain resistant overcoat are rather unique. The stain resistant layer must not effect the transparency, color, or other imaging properties of the film. The application and curing of the stain resistant coating must be compatible with the imaging element manufacturing process. The overcoat layer must provide stain resistance when applied as a submicron-thick layer and protect against common stains such as grease and food and drink products, as well as stains specific to the imaging industry such as from the deposition of tar present in film processing tanks.
The present invention relates to improving the stain resistance of imaging elements by providing a thin, outermost layer that prevents tar pickup during film processing and resists permanent staining by dirt, grease, food and drink products, etc.
Further, the stain resistant outermost layer of the invention does not degrade the transparency, frictional characteristics, or other physical properties of the imaging element, and may be applied from solvent or aqueous media at low cost.